Up to now, the analysis has rarely been conducted of thermal-mechanical mirror buckling behavior of freestanding graphene membranes discovered in scan tunneling microscope experiments. One of the potential applications of the out-of-plane deformational behavior of graphene membranes is energy harvesting system. Whether in the experiments or for energy harvesting systems, the size of graphene membrane needs to be down to micron scale. According to previous researches, traditional molecular dynamics method is a suitable method to characterize nano-scale mirror buckling. However, owing to the limit of algorithm, when dealing with micro size model by molecular dynamics method, two problems arise: low computational efficiency and too long calculation time. Therefore, for analyzing the mirror buckling of micro size graphene membranes, the coarse-grained molecular dynamics method is utilized in this work. Graphene membranes with a fan-shaped cross section and various depth-span ratios are under mechanical or thermal loads. Effects of each factor on the mirror buckling are investigated. The calculations indicate that for graphene membranes with various depth-span ratios under mechanical load mirror buckling can be observed. And the critical loading increases with the depth-span ratio increasing. Under thermal load graphene membranes only with low depth-span ratios can undergo complete flipping phenomenon. For high depth-span ratio graphene, the center height decreases with temperature rising. However, it is hard to flip over completely. The understanding of the effects of various factors on the mirror buckling phenomenon of graphene membranes can provide theoretical guidance for designing the energy harvesting systems.